1. Elementary number theory only needs middle school knowledge as preparatory knowledge.
2. Before learning Analytic Number Theory and Algebraic Number Theory, it is necessary to complete most of the specialized courses of mathematics department from undergraduate to graduate.
3. Algebra number theory may require advanced algebra for undergraduate students, abstract algebra, commutative algebra for graduate students, topology, algebraic topology and algebraic geometry. With these, you can begin to understand.
4. If you want to analyze number theory, you need undergraduate mathematical analysis calculus, real variable function, complex variable function, Fourier analysis and some algebraic foundations. You also need (simple) complex analysis of graduate students (which is very closely related), and maybe a little real analysis.